1955 Had a wrap around back glass, 1965 had either a small or big back glass, the big back window was called the Extra View on the GMC options list, and it was NOT part of the custom package, you had to pay extra for it, about $40.

1965 GMC 3500 Rescue in the Stovebolt Gallery

My Facebook Links:
Jolly Goodfellow.
the 6066 GMC Guy.
Jolly's Truck & Car Show Page.
Owner of ~ 1965 GMC 3500 Custom Rescue Squad {Black Built Bodies}.